Wireless communication networks are well known. Some networks are proprietary, while others are compliant with one or more standards that allow various vendors to manufacture equipment for a common system. One such standards-based network is the Universal Mobile Telecommunications System (UMTS). UMTS is standardized by the Third Generation Partnership Project (3GPP), which is a collaboration of groups of telecommunications associations to make a globally applicable Third Generation (3G) mobile phone system specification within the scope of the International Mobile Telecommunications-2000 project of the International Telecommunication Union (ITU). Efforts are currently underway to develop an evolved UMTS standard, which is typically referred to as UMTS Long Term Evolution (LTE) or Evolved UMTS Terrestrial Radio Access (E-UTRA).
According to Release 8 of the E-UTRA or LTE standard or specification, downlink communications from a base station (referred to as an “enhanced Node-B” or simply “eNB”) to a wireless communication device or terminal (referred to as “user equipment” or “UE”) utilize orthogonal frequency division multiplexing (OFDM). In OFDM, orthogonal subcarriers are modulated with a digital stream, which may include data, control information, or other information, so as to form a set of OFDM symbols. The subcarriers may be contiguous or non-contiguous and the downlink data modulation may be performed using quadrature phase shift-keying (QPSK), 16-ary quadrature amplitude modulation (16QAM), or 64QAM. The OFDM symbols are configured into a downlink subframe for transmission from the base station. Each OFDM symbol has a time duration and is associated with a cyclic prefix (CP). A cyclic prefix is essentially a guard period between successive OFDM symbols in a subframe. According to the E-UTRA specification, a normal cyclic prefix is about five (5) microseconds and an extended cyclic prefix is 16.67 microseconds.
In contrast to the downlink, uplink communications from the UE to the eNB utilize single-carrier frequency division multiple access (SC-FDMA) according to the E-UTRA standard. In SC-FDMA, block transmission of QAM data symbols is performed by first discrete Fourier transform (DFT)-spreading (or precoding) followed by subcarrier mapping to a conventional OFDM modulator. The use of DFT precoding allows a moderate cubic metric/peak-to-average power ratio (PAPR) leading to reduced cost, size and power consumption of the UE power amplifier. In accordance with SC-FDMA, each subcarrier used for uplink transmission includes information for all the transmitted modulated signals, with the input data stream being spread over them. The data transmission in the uplink is controlled by the eNB, involving transmission of scheduling requests (and scheduling information) sent via downlink control channels. Scheduling grants for uplink transmissions are provided by the eNB on the downlink and include, among other things, a resource allocation (e.g., a resource block size per one millisecond (ms) interval) and an identification of the modulation to be used for the uplink transmissions. With the addition of higher-order modulation and adaptive modulation and coding (AMC), large spectral efficiency is possible by scheduling users with favorable channel conditions.
E-UTRA systems also facilitate the use of multiple input and multiple output (MIMO) antenna systems on the downlink to increase capacity. As is known, MIMO antenna systems are employed at the eNB through use of multiple transmit antennas and at the UE through use of multiple receive antennas. A UE may rely on a pilot or reference symbol (RS) sent from the eNB for channel estimation, subsequent data demodulation, and link quality measurement for reporting. The link quality measurements for feedback may include: such spatial parameters as rank indicator or the number of data streams sent on the same resources; precoding matrix index (PMI); and rank indicator (RI) and coding parameters, such as a modulation and coding scheme (MCS) or a channel quality indicator (CQI). Together MCS or CQI, PMI and RI constitute elements of the Channel State Information (CSI) which convey the quality of the MIMO channel indicative of the reliability and condition number of the channel capable of supporting multi-stream communication between the eNB and the UE. For example, if a UE determines that the link can support a rank greater than one, it may report multiple CQI values (e.g., two CQI values when rank=2 by signaling of the corresponding RI). Further, the link quality measurements may be reported on a periodic or aperiodic basis, as instructed by an eNB, in one of the supported feedback modes. The reports may include wideband or subband frequency selective information of the parameters. The eNB may use the rank information, the CQI and other parameters, such as uplink quality information, to serve the UE on the uplink and downlink channels.
E-UTRA systems must be compliant to regulatory requirements on spurious emissions on licensed bands in different regions of the world. E-UTRA follows the “uplink after downlink” principle which means that a UE must transmit on its uplink only when its downlink is reliable. In other words, a UE that does not have a reliable downlink must continuously monitor the quality of the downlink signal by tracking the downlink signal quality (e.g., based on channel state estimation) and stop transmission on its uplink if the downlink signal quality falls below a threshold. In E-UTRA, this is enabled by means of Radio Link Monitoring (RLM) UE procedures where a UE continuously monitors a cell-specific reference signal (CRS) on the downlink and determines the channel state (including estimating the propagation channel between the eNB and the UE and the underlying interference on the same carrier). Qout is defined as the condition that the channel quality between eNB and the UE is such that the Block Error Rate (BLER) of a first hypothetical PDCCH transmission exceeds 10%. This event is also denoted as an “out-of-sync” event. Qin is defined as the condition that the channel quality between the eNB and the UE is such that the BLER of a second hypothetical PDCCH transmission drops below 2%. This event is also denoted as an “in-sync” event. The UE monitors the channel state in RRC_CONNECTED mode continuously or periodically in both non-discontinuous reception (non-DRX) and discontinuous reception (DRX) states to evaluate whether Qout or Qin has occurred. Upon several successive Qout detections, the UE must determine that a Radio Link Problem (RLP) has occurred. In the RLP state, the UE must assume that it has lost its downlink with the serving eNB and start monitoring the link for recovery. If a Qin is detected within a certain time duration as configured by the eNB by means of a Radio Resource Control (RRC) timer, the UE resumes normal RRC_CONNECTED operation. On the other hand, if a Qin is not detected within the time duration, the UE must determine that a Radio Link Failure (RLF) has occurred and must stop all uplink transmission within 40 ms. The RLM procedure reduces the probability that a UE jams the uplink of a neighbor cell when the UE has lost the serving cell downlink but has not been handed over by the network to a different cell due to Radio Resource Management (RRM) inefficiencies.
Like other 3GPP standards, E-UTRA supports mobility of UEs by RRM measurements and associated support for RRC signaling including specified eNB and UE behavior in both RRC_CONNECTED and RRC_IDLE states. In the RRC_CONNECTED state, a UE can be configured to measure and report Reference Signal Received Power (RSRP) and Reference Signal Received Quality (RSRQ) for both the serving cell and the neighbor cells (on the serving cell carrier and inter-frequency carriers). A network element such as the eNB or the Mobility Management Entity (MME) can perform UE handovers based on the reported measurements. In RRC_IDLE state, the UE can be configured to measure RSRP and RSRQ and perform cell reselections based on these measurements.
Heterogeneous networks comprise a variety of base stations serving mobile stations. In some systems, the base stations operate on the same carrier frequency. The variety of base stations can include some or all of the following types of base stations: conventional macro base stations (macro cells), pico base station (pico cells), relay nodes and femto base stations (also referred to as femto cells, closed subscriber group (CSG) cells or Home eNodeBs). Macro cells typically have coverage areas that range from several hundreds of meters to several kilometers. Pico cells, relays and femto cells can have coverage areas that are considerably smaller than the coverage area of typical macro cells. Pico cells can have coverage areas of about 100-200 meters. Femto cells are typically used for indoor coverage, and can have coverage areas in the 10 s of meters. Relay nodes are characterized by a wireless backhaul to a donor base station, and can have coverage areas similar to pico cells.
A home-base station or femto-cell or pico-eNB or relay node (RN) is referred to as hetero-eNB (HeNB) or a hetero-cell or hetero base station in the sequel. A HeNB can either belong to a CSG as mentioned earlier or can be an open-access cell. HeNBs are used for coverage in a small area (such as a home or office) in contrast with eNBs (also referred to as macro eNBs or macro-cells) which are typically used for coverage over a large area. A CSG is set of one or more cells that allow access only to a certain group of subscribers. HeNB deployments where at least a part of the deployed bandwidth (BW) is shared with macro-cells are considered to be high-risk scenarios from an interference point-of-view. When UEs connected to a macro-cell roam close to a HeNB, the uplink of the HeNB can be interfered with particularly when the HeNB is far away (for example >400 m) from the macro-cell, thereby, degrading the quality of service of UEs connected to the HeNB. The problem is particularly troublesome if the UE is not allowed to access the HeNB that it roams near (for example, due to the UE not being a member of the CSG of the HeNB). Currently, the existing LTE Rel-8/9 UE measurement framework can be made use of to identify the situation when this interference might occur and the network can handover the UE to an inter-frequency carrier which is not shared between macro-cells and HeNBs to mitigate this problem. However, there might not be any such carriers available in certain networks over which to handover the UE. Further, as the penetration of HeNBs increases, being able to efficiently operate HeNBs on the entire available spectrum might be desirable for maximizing spectral efficiency and reducing overall operational cost. Several other scenarios are likely too, including the case of a UE connected to a HeNB experiencing interference from an adjacent HeNB or a macro cell. The following types of interference scenarios have been identified.                HeNB (aggressor)→MeNB (victim) downlink (DL)        HUE (aggressor)→MeNB (victim) uplink (UL)        MUE (aggressor)→HeNB (victim) UL        MeNB (aggressor)→HeNB (victim) DL        HeNB (aggressor)→HeNB (victim) on DL        HeNB (aggressor)→HeNB (victim) on UL.        
Heterogeneous networks can potentially enable an operator to provide improved service to users (e.g., increased data rates, faster access, etc.) with lower capital expenditure. Typically, installation of macro base stations is very expensive as they require towers. On the other hand, base stations with smaller coverage areas are generally much less expensive to install. For example, pico cells can be installed on roof tops and femto cells can be easily installed indoors. The pico and femto cells allow the network to offload user communication traffic from the macro cell to the pico or femto cells. This enables users to get higher throughput and better service without the network operator installing additional macro base stations or provisioning more carrier frequencies for communication. Thus, heterogeneous networks are considered to be an attractive path for evolution of wireless communication networks. 3GPP has commenced work on enabling heterogeneous E-UTRA networks in 3GPP LTE Rel-10.
FIG. 1 illustrates an E-UTRA Heterogeneous network comprising a macro cell, pico cells and femto cells operating on a single carrier frequency. A mobile station, also referred to as “user equipment” (UE), may be associated with one of the cells based on its location. The association of a UE to a cell can refer to association in idle mode or connected mode. That is, a UE is considered to be associated with a cell in idle mode if it is camped on the cell in idle mode. Similarly, a UE is considered to be associated with a cell in connected mode if it is configured to perform bi-directional communication with a cell (for example, a UE in E-UTRA radio resource control (RRC) connected mode can be connected to and therefore associate with a cell). A UE associated with a macro cell is referred to as a macro UE, a UE associated with a pico cell is referred to as a pico UE, and a UE associated with a femto cell is referred to as a femto UE.
Various time-division approaches are possible for ensuring that base stations in heterogeneous networks share the frequency spectrum while minimizing interference. Two approaches can be envisioned. A network can configure time periods when different base stations are required to not transmit. This enables cells that can interfere with one another to transmit in mutually exclusive time periods. For example, a femto cell can be configured with some time periods during which it does not transmit. If a macro UE is located within the coverage of the femto cell, the macro cell can use the time periods during which the femto cell does not transmit data to the UE.
The network can configure time periods where a first base station transmits on all available time periods (e.g., pico eNBs), while a second base station (e.g., macro eNB) transmits only on a subset of the available time periods. A UE connected to the first base station can therefore have two “virtual” channels at different channel qualities depending on how much the second base station's transmission interferes with that for the first (i.e., signal geometry of the first base station relative to the second). The first virtual channel is where only the first base station transmits data while the second base station does not transmit data. The second virtual channel is one where both the first and the second base stations transmit data. The first base station can use adaptive modulation and coding and schedule at different MCS levels on the two virtual channels. In the extreme case, the first base station may not schedule at all on the second virtual channel when interference from the second base station is large.
However, it should be noted that time division approaches can lead to inaccurate or inconsistent RRM, RLM and CSI measurements. For example, if a macro UE located near a femto cell performs measurements during time periods when the femto cell transmits, the measured values can be significantly different from measured values obtained from measurements made during time periods when the femto cell does not transmit. Such measurements can lead to erratic behaviors, such as failed connections, unnecessary handovers and unnecessary cell reselections. In addition, such inaccuracies can lead to suboptimal scheduling on UE downlink leading to inefficient utilization of spectral resources. Therefore, methods are needed for performing measurements of cells that overcome the problems mentioned above.
The various aspects, features and advantages of the disclosure will become more fully apparent to those having ordinary skill in the art upon a careful consideration of the following Detailed Description thereof with the accompanying drawings described below. The drawings may have been simplified for clarity and are not necessarily drawn to scale.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale or to include every component of an element. For example, the dimensions of some of the elements in the drawings may be exaggerated alone or relative to other elements, or some and possibly many components of an element may be excluded from the element, to help improve the understanding of the various embodiments of the disclosure.